The need for reducing the amount of data required to reproduce still and full motion color video images has been recognized. For example, the invention disclosed in U.S. Pat. No. 4,868,653, assigned to the assignee of the present application, is directed to meeting the need for a compression system for providing a compressed digital video signal representative of a full motion color video signal, which is suitable for recording or transmission using relatively narrow band media and which may be decompressed at speeds at least equal to conventional video frame rates.
Ideally such compression techniques read a sequence and produce a bitstream that satisfies the bit rate and decode time constraints. The compression system can adjust to gradually changing image complexity and to abrupt scene changes. However, images often break up during periods of rapid motion. This is because the automated system reacts to increased image complexity by loosening tolerances. This causes image quality to deteriorate, but has been deemed necessary to control the bit rate. It was subsequently determined that such is not always necessary; and, by inclusion of a sophisticated buffering scheme, such a system would be able to ride out a temporary bit-rate overload.
Unfortunately, the compression system does not know whether the overload is temporary until it is too late. Therefore, it does not know whether to react slowly or rapidly to the overload. If it reacts too slowly to a major increase in image complexity, a serious overload will develop. On the other hand, if the system reacts too quickly, the resulting difficult scenes are of poor quality. In addition, an instability is possible where the tolerances oscillate long after the image complexity has stabilized.